Transformation process of Al-Cu-Li alloy sheets

Abstract

The invention concerns a process to manufacture a flat-rolled product, notably for the aeronautic industry containing aluminum alloy comprising 2.1% to 3.9% Cu by weight, 0.7% to 2.0% Li by weight, 0.1% to 1,0% Mg by weight, 0% to 0.6% Ag by weight, 0% to 1% Zn by weight, at least 0.20% Fe+Si by weight, at least one element chosen from Zr, Mn, Cr, Sc, Hf and Ti, the quantity of said element, if chosen, being 0.05% to 0.18% by weight for Zn, 0.1% to 0.6% by weight for Mn, 0.05% to 0.3% by weight for Cr, 0.02% to 0.2% by weight for Sc, 0.05% to 0.5% by weight for Hf and 0.01% to 0.15% by weight for Ti, the other elements at most 0.05% by weight each and 0.15% by weight in total, the rest being aluminum, in which, notably a flattening and / or stretching is performed with a cumulated deformation of at least 0.5% and less than 3%, and a short heat-treatment is performed in which the sheet reaches a temperature between 130° C. and 170° C. for a period of 0.1 to 13 hours. The invention notably makes it possible to simplify the forming process of fuselage skins and to improve the balance between static mechanical strength properties and damage tolerance properties.

Description

CROSS REFERENCE TO RELATED APPLICATIONS[0001]This application claims the priority to French Application No. 1103155, filed Oct. 14, 2011, and U.S. Provisional Application No. 61 / 547,289, filed Oct. 14, 2011, the contents of both of which are incorporated herein by reference in their entireties.BACKGROUND OF THE INVENTION[0002]1. Field of the Invention[0003]The invention relates to aluminum-copper-lithium alloy products, and more particularly to such products, their manufacturing processes and use, designed in particular for aeronautical and aerospace engineering.[0004]2. Description of Related Art[0005]Flat-rolled products made of aluminum alloy are developed to produce parts of high strength designed for the aircraft and aerospace industry in particular.[0006]Aluminum alloys containing lithium are of great interest in this respect because lithium can reduce the density of aluminum by 3% and increase the modulus of elasticity by 6% for each percent of lithium weight added. For these...

AI Technical Summary

Benefits of technology

The invention is a method for producing a sheet with improved mechanical properties for use in artificial aging. The method involves a short heat-treatment followed by additional cold working, which can reach values of 6% to 8% or even 10% in local or global deformation. The cold working can be performed through various forming processes such as drawing, stretch-forming, stamping, spinning, or bending. The resulting sheet has high tensile yield strength and does not decrease in toughness even with additional deformation. The method can be carried out on the sheet manufacturer's premises or directly on the product delivered to the aeronautic structure manufacturer. The sheet produced using this method has a combination of static mechanical properties and toughness properties that meet the requirements for use in artificial aging.

Problems solved by technology

Generally speaking, after solution heat-treatment and quenching, the sheets are in a state characterized by good formability, although this state is unstable (“W” temper), and forming must take place in an as-quenched condition, i.e. inside a brief delay after the quench, from roughly ten minutes to a few hours.

If this is not possible for production management reasons, the sheet must be stored in a cold room at a sufficiently low temperature and for a sufficiently short duration to avoid natural maturation.

In certain cases, it is noted that for excessively short durations after solution heat-treatment, Lüders lines appear after forming, which requires an additional requirement with a minimum waiting period.

For voluminous and highly formed parts, this solution heat-treatment requires large-scale furnaces, which makes the operation cumbersome, including in relation to the same operation performed on flat sheet.

The possible need for a cold room adds to the costs and drawbacks of the prior art.

In addition, the sheet may be deformed after quenching and create problems associated with this deformation, for example, when positioning it in the jaws of the stretch-forming tool.

Forming complex structural elements in a T8 temper is limited to mild forming because elongation and the ratio Rm / Rp0,2 are too low in this temper.

It is generally accepted that complex deformations after solution heat-treatment and quenching lead to an increase in mechanical strength but with a sharp deterioration in toughness.

It was further found that the total energy at break measured by Kahn test which is connected to toughness decreases with deformation and with a more brutal decrease for 6% strain, which poses the problem of obtaining high toughness regardless of the rate of local deformation during forming.

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